1. Field of the Invention
The present invention relates to a storage disc, particularly to a method of correcting nonalignment between disc-shaped resin substrates each constituting a storage disc.
2. Prior Art
Computers, especially personal computers, have become remarkably popular recently, and the capacity of the storage medium to be used thereby, particularly the capacity of a storage disc and the kinds of storage media have increased. In addition, the application of storage discs to various fields has also increased. Accordingly, in a CD mainly for use in music, there is a tendency to develop the storage disc as a video disc.
The storage disc may be in the form of a magnetic disc, an optical disc (e.g., CD-ROM), an optical magnetic disc (e.g., MO), etc. The demand for optical discs as storage, discs has increased recently.
Exemplifying an optical disc called a DVD, a disc-shaped resin substrate that is a single plate constituting the DVD is required to have a thickness of 0.6 mm, an outer diameter of 120 mm, and a central hole inner diameter of 15 mm. If such a disc-shaped resin substrate is formed of a single substrate, it is low in a mechanical strength and is easily deformable. Accordingly, two disc-shaped resin substrates each having the same thickness (0.6 mm) are bonded to each other to form an integrated substrate.
For example, FIGS. 11(A) and (B) schematically generally show a DVD serving as an optical disc which DVD is formed of two disc-shaped resin substrates (a first disc-shaped resin substrate D1 and a second disc-shaped resin substrate D2) which are bonded to each other.
An Information signal is applied to one disc-shaped resin substrate D1, namely information is stored in one disc-shaped resin substrate as shown in FIG. 11(A), while information is stored in both disc-shaped substrates D1 and D2 as shown in FIG. 11(B).
An optical disc D formed by bonding two disc-shaped resin substrates D1 and D2 receives light reflected from the reflection film D11 using a laser beam by a light detector, not shown, to thereby reproduce signals.
Since the high density storage disc including the DVD does not include a single substrate or plate, but frequently includes plural substrates or plates, the first disc-shaped resin substrate D1 and the second disc-shaped resin substrate D2 must be bonded to each other.
The following series of steps are performed to form an integrated storage disc (i.e., an optical disc) by bonding each single plate (see FIG. 12).
Step 1: placing the first disc-shaped resin substrate D1 on a rotary holding table C;
Step 2: coating an adhesive R onto the first disc-shaped resin substrate D1;
Step 3: placing the second disc-shaped resin substrate D2 on the first disc-shaped resin substrate D1;
Step 4: developing the adhesive R interposed between the first and second disc-shaped resin substrates D1 and D2; and
Step 5: curing the thus developed adhesive R.
These steps are explained in more detail herebelow.
In Step 1, the first disc-shaped resin substrate D1 having thereon an information storage surface coated with a reflection film and a protection film is uniformly drawn by and held on the rotary holding table C.
In Step 2, the adhesive R, e.g., UV curing resin is slowly discharged from a discharge nozzle N while the rotary holding table C on which the first disc-shaped resin substrate D1 is placed is rotated at low speed (several ten rpm). The adhesive R has a different track on the first disc-shaped resin substrate D1 depending on a manner in which it is discharged from the discharge nozzle N, but it is preferable to have a doughnut-shaped track as shown in FIGS. 11(A) and 11(B).
In Step 3, a transparent second disc-shaped resin substrate D2 is placed on the first disc-shaped resin substrate D1 which is coated with the adhesive R. The substrate D2 may be in the form of a transparent substrate on which no information signal is applied, or the substrate D2 may be in the form of a substrate on which an information signal is applied.
In Step 4, the adhesive R interposed between the first and second disc-shaped resin substrates D1 and D2 is developed to extend uniformly between the first and second disc-shaped resin substrates D1 and D2.
This development of the adhesive R is performed by rotating the rotary holding table C at high speed (normally, several thousands rpm or more for about several seconds) in a state where the storage disc D, namely, the integrated disc-shaped resin substrate formed by bonding the first and second disc-shaped resin substrates D1 and D2, is placed on the rotary holding table C.
When the rotary holding table C is rotated at high speed, surplus adhesive R which is present between the bonded first and second disc-shaped resin substrates D1 and D2 is scattered outside while it is developed, and air (air bubble, etc.) confined between the first and second disc-shaped resin substrates D1 and D2 is discharged outside so that the adhesive R can be uniformly developed between the first and second disc-shaped resin substrates D1 and D2. During the development of the adhesive R, the adhesive R is drawn in the direction of the center through the boss of the rotary holding table which is inserted into the central hole of the storage disc.
In Step 5, the optical disc is irradiated with UV rays, namely, after the first and second disc-shaped resin substrates D1 and D2 are irradiated with UV rays in a state where the optical disc is slowly rotated (e.g. at about several rpm which is much slower than the rpm in the developing state set forth above) or not rotated so that the adhesive R, e.g., a UV curing resin layer is cured.
More specifically, the adhesive R is irradiated with UV light source UL provided with a reflection mirror at the back side thereof, thereby effectively curing the adhesive.
Further, the curing step is differs due to the kind of the adhesive R to be used, and hence it should be understood that a curing method conforming to the characteristic of the adhesive to be used is employed. In such a manner, the bonding step is completed.
In disc-shaped resin substrates of the type used to store information, such as substrates D1 and D2, there are two methods used to obtain information from the storage disc. The first method includes reading the information or signal by applying light from one direction (a method of reading information from a storage disc of a type having a standard used by a dual layer DVD), and the second method includes reading information by applying light from both directions (a method of reading information from a storage disc used by a single layer or double-sided DVD). In the first method, if there is a misalignment between the disc-shaped resin substrates D1 and D2, information contained in the disc-shaped resin substrates D1 and D2 cannot be correctly read if the central holes of the substrates are not concentrically positioned.
FIG. 10 shows a state where the nonalignment occurs between the disc-shaped resin substrates D1 and D2 and wherein slippage of information stored in the disc-shaped resin substrates D1 and D2 also occurs. If the disc-shaped resin substrates D1 and D2 are bonded with each other in this state, the information cannot be correctly read from one direction.
Meanwhile, in a standard of dual layer DVD, it is preferable that the nonalignment between the disc-shaped resin substrates D1 and D2 does not exceed 15 .mu.m to prevent the DVD from becoming a defective product. The nonalignment has already occurred in a state where the disc-shaped resin substrate D2 is placed on the disc-shaped resin substrate D1.
In the next developing step, the adhesive is developed on the rotary holding table on which the disc-shaped resin substrates D1 and D2 are placed while there occurs the nonalignment or misalignment therebetween.
In the developing step, the boss 1 on the rotary holding table is inserted into the central holes of the disc-shaped resin substrates D1 and D2 to temporarily position the disc-shaped resin substrates D1 and D2. Since there is a slight difference between the outer diameter of the boss 1 and the diameters of the central holes of the disc-shaped resin substrates D1 and D2, a minute nonalignment between the disc-shaped resin substrates D1 and D2 cannot be corrected even when they are positioned by the boss 1.
Accordingly, the nonalignment between the disc-shaped resin substrates D1 and D2 is not corrected but remains as it is, even if the developing step of the adhesive is completed.
Although the adhesive is cured on the entire surfaces on the disc-shaped resin substrates D1 and D2 in the curing step upon completion of the developing step, the nonalignment remains as it is, namely, it is fixed when the adhesive is cured, which permits the nonalignment to remain on the final product.
Under the circumstances, it is necessary that the nonalignment between the disc-shaped resin substrates D1 and D2 must be removed or corrected to the utmost before the curing step occurs.